Generally, robots refer to controllably deformable structures for accomplishing certain tasks, as well as to master arms actuated by an operator for transmitting instructions to a slave arm reproducing the movements of the master arm or to a virtual environment via mechanical transmissions or computer interfaces.
Several design constraints weigh heavily on robots and notably on master arms. First of all, the operator of a master arm must experience sensations analogous to those which he would have experienced by directly controlling the slave arm, especially when the latter is working: the stresses received by the slave arm should therefore be reproduced with sufficient accuracy in the master arm. The linkages between the different portions of the robot, essentially the joints, should be provided with little play and fitted out with accurate low inertia transmissions, having little friction but capable of being easily blocked. Finally, it is desirable that the robots be as lightweight as possible which involves not only reducing their own weight but distributing it so that it is only moderately exerted on the control means of the robot used for moving it or, on the contrary, for maintaining it in a stable state by recovering static stress.
Above all, the means used for thereby controlling the joints or the other linkages of the robot, comprise motors which are among the heaviest components of the robot. It is current practice to position them as much as possible on the bases or low parts of the robot in order to reduce or suppress the moments required for lifting or moving them. It is then mandatory to provide a suitable transmission between the motor and the unit of the robot which it drives and which also provides reduction of the angular velocity of the motor. This is easy if the unit is directly connected to the base; but if it is connected to the base via another unit, mobile on the base, and therefore providing another degree of freedom, it is much more difficult to design a suitable transmission because of the much more complex positions which the unit may take up relatively to the base.
The tensioned cables between the shaft of the motor and a fixing portion belonging to the driven unit should be mentioned among the transmissions which may be proposed. Such transmissions are adopted on remote manipulators with master and slave arms, where the cables however suffer from the drawback of being long, which reduces the stiffness of the transmission and of passing through complex trajectories which cause couplings between the movements of different parts of the robot.
The transmission cables positioned between two robot portions which are not directly connected with each other should normally be tensioned on pulleys fixed on intermediate units. Then there occur problems in that the distances between their fixing points on the motor, the intermediate units and the unit which they drive, generally vary, thereby-producing a change in the cable's tension through elasticity, with the drawback that the robot's stiffness is changed. The harm is more marked when the robot is a master arm which is displaced by hand and the motor is a force feedback motor, because the operator can only be uncomfortable by feeling that the mechanical strength opposed by the arm varies with the displacement.
An improved cable reducer drive is provided as an essential component of the invention. In its most general embodiment, it relates to an articulated mechanism which may belong to a robot arm, comprising a base, a support rotating on the base around a first axis and a unit rotating on the support around a second axis, which is not parallel to the first axis, as well as a unit actuator for having it rotate, the actuator comprising: a motor secured to the base; and characterized in that the actuator further comprises a tensioned cable between a shaft of the motor, at least a pair of idle pulleys rotating on the support and a pulley connected to the, unit, with the idle pulleys being substantially tangent to the first axis; the cable forming a pair of strands substantially in extension and collinear with the first axis; the idle pulleys and the pulley of the unit being positioned so that said strands of the pair extend between the pulley of the unit and the idle pulleys, respectively.
By having the cable form a pair of strands substantially in extension and collinear with the first axis, it is guaranteed that the extension and the tension of the cable are only very slightly changed when the intermediate support rotates around the first axis. Advanced designs of the mechanism further enable this undesirable extension to be reduced.
According to certain advantageous features of the invention, the shaft of the motor is perpendicular to the axes of rotation of two of the idle pulleys between which and the shaft of the motor, the cable forms two rectilinear strands; and a linear transmission exists between the shaft of motor and the cable.
An articulated mechanism which may belong to a robot arm, comprising a base, a support rotating on the base around a first axis, is provided in a more complex embodiment of the invention, characterized in that it comprises two pulleys for controlling an arm through a linkage mechanism and two actuators of the control pulleys, the actuators each comprising: a motor secured on the base; a tensioned cable between a shaft of the motor, a pair of idle pulleys rotating on the support and one of the control pulleys; the idle pulleys being substantially tangent to the first axis; the idle pulleys and the control pulleys being positioned so that the cables form pairs of strands substantially in extension and collinear with the first axis which each extend between one of the control pulleys and one of the idle pulleys; advantageously, the link mechanism comprises a unit which is stiffly connected to one of the control pulleys and an articulated connecting rod at the other of the control pulleys, the arm being articulated at the connecting rod and at the unit; the control pulleys being parallel.